As the others mentioned, you get certain discounts and perks. If you're there for the holiday season, you get this awesome packet of discounts, comp tickets, and coupons. I got it both times I worked for the company, so I can vouch for that.
I will say, though, that if you're trying to get into a park on a holiday and it's just you and you're going in through the cast entrance, you will be asked if you're working. If you say no, security will tell you to leave and go through the guest entrance. That's particularly important on days like July 4th, Christmas, and New Year's. The reason they tell you to go through the guest entrance is because of safety codes. If a hundred CM's go through the cast entrance and the park's at capacity, it's a fire hazard.